Role of subtyping in detecting Salmonella cross contamination in the laboratory

BACKGROUND: With the exception of M. tuberculosis, little has been published on the problems of cross-contamination in bacteriology laboratories. We performed a retrospective analysis of subtyping data from the National Salmonella Reference Laboratory (Ireland) from 2000–2007 to identify likely incidents of laboratory cross contamination. METHODS: Serotyping and antimicrobial susceptibility testing was performed on all Salmonella isolates received in the NSRL. Phage typing was performed on all S. Typhimurium and S. Enteritidis isolates while multi-locus variance analysis (MLVA) was performed on selected S. Typhimurium isolates. Pulsed field gel electrophoresis (PFGE) using the PulseNet standard protocol was performed on selected isolates of various serovars. RESULTS: Twenty-three incidents involving fifty-six isolates were identified as likely to represent cross contamination. The probable sources of contamination identified were the laboratory positive control isolate (n = 13), other test isolates (n = 9) or proficiency test samples (n = 1). CONCLUSION: The scale of laboratory cross-contamination in bacteriology is most likely under recognized. Testing laboratories should be aware of the potential for cross-contamination, regularly review protocols to minimize its occurrence and consider it as a possibility when unexpected results are obtained.